Copyright 2005 John Wiley & Sons, Inc5 - 1 Business Data Communications and Networking 8th Edition Jerry Fitzgerald and Alan Dennis John Wiley & Sons, Inc Prof. M. Ulema Manhattan College Computer Information Systems

Copyright 2005 John Wiley & Sons, Inc5 - 2 Chapter 5 Network and Transport Layers

Copyright 2005 John Wiley & Sons, Inc5 - 3 Outline Transport & Network Layer Protocols –TCP/IP, IPX/SPX, X.25, SNA Transport Layer Functions –Interacting with Application Layer –Packetizing –End-to-en delivery of application layer messages Network Layer Functions –Addressing –Routing TCP/IP Examples

Copyright 2005 John Wiley & Sons, Inc5 - 4 Introduction Transport and Network layers –Responsible for moving messages from end-to-end in a network –Closely tied together –TCP/IP: most commonly used protocol Used in Internet Compatible with a variety of Application Layer protocols as well as with many Data Link Layer protocols Network Layer Data Link Layer Application Layer Transport Layer

Copyright 2005 John Wiley & Sons, Inc5 - 5 Introduction - Transport layer Responsible for end-to-end delivery of messages –Sets up virtual circuits (when needed) Responsible for segmentation and reassembly –Breaking the message into several smaller pieces at the sending end –Reconstructing the original message into a single whole at the receiving end Interacts with Application Layer Transport Layer Application Layer Network Layer

Copyright 2005 John Wiley & Sons, Inc5 - 6 Introduction – Network Layer Responsible for addressing and routing of messages –Selects the best path from computer to computer until the message reaches destination Performs encapsulation on sending end –Adds network layer header to message segments Performs decapsulation on receiving end –Removes the network layer header at receiving end and passes them up to the transport layer Network Layer Transport Layer Data Link Layer

Copyright 2005 John Wiley & Sons, Inc5 - 7 TCP/IP’s 5-Layer Network Model

Copyright 2005 John Wiley & Sons, Inc5 - 8 Transport/Network Layer Protocols TCP/IP (Transmission Control Protocol / Internet Protocol) –Most common, used by all Internet equipment IPX/SPX –Similar to TCP/IP –Mainly used by Novell networks (Novell has since replaced it with TCP/IP) X.25 –Used mainly in Europe SNA (System Network Architecture) –IBM’s protocol suite

Copyright 2005 John Wiley & Sons, Inc5 - 9 TCP/IP Developed in ‘74 by V. Cerf and B. Kahn –As part of Arpanet (U.S. Department of Defense) Most common protocol suite –Used by the Internet. –Almost 70% of all backbone, metropolitan, and wide area networks use TCP/IP –Most common protocol on LANs (surpassed IPX/SPX in ‘98) Reasonably efficient and error free transmission –Performs error checking –Transmits large files with end-to-end delivery assurance –Compatible with a variety of data link layer protocols

Copyright 2005 John Wiley & Sons, Inc Transmission Control Protocol TCP Header: 192 bits (24 bytes)used in message reassembly Links the application layer to the network layer Performs packetization and reassembly Breaking up a large message into smaller packets Numbering the packets and Reassembling them at the destination end Ensures reliable delivery of packets

Copyright 2005 John Wiley & Sons, Inc Internet Protocol (IP) Responsible for addressing and routing of packets Two versions in current in use –IPv4: a 192 bit (24 byte) header, uses 32 bit addresses. –IPv6: Mainly developed to increase IP address space due to the huge growth in Internet usage (128 bit addresses) Both versions have a variable length data field –Max size depends on the data link layer protocol. –e.g., Ethernet’s max message size is 1,492 bytes, so max size of TCP message field: 1492 – 24 – 24 = 1444 bytes TCP header IPv4 header

Copyright 2005 John Wiley & Sons, Inc IP Packet Formats IPv4 Header: 192 bits (24 bytes) IPv6 Header: 320 bits (40 bytes)

Copyright 2005 John Wiley & Sons, Inc X.25 Developed by ITU-T for use in WANs Widely used especially in Europe –Seldom used in North America Transport layer protocols for X.25 –X.3 (performs packetization for ASCII terminals) –TP (ISO defined), TCP Network Layer protocol for X.25 –Packet Layer Protocol (PLP) for routing and addressing Data Link Layer protocol for X.25 –LAP-B (Link Access Protocol-Balanced) Recommended packet size: 128 bytes –But can support packet sizes up to 1024 bytes.

Copyright 2005 John Wiley & Sons, Inc SNA - Systems Network Architecture Developed by IBM –Used on IBM and IBM-compatible mainframes Based on non-standard proprietary protocols –Difficult to integrate with non-SNA networks –Requires special equipment, gateways (to route messages between SNA and non-SNA networks) Likely disappear over time –IBM now offers TCP/IP on its networks

Copyright 2005 John Wiley & Sons, Inc Transport Layer Functions Linking to Application Layer Packetization and Reassembly Establishing connection (virtual) –Connection Oriented –Connectionless –Quality of Service (QoS)

Copyright 2005 John Wiley & Sons, Inc Linking to Application Layer TCP may serve several Application Layer protocols at the same time –Problem: Which application layer program to send a message to? –Solution: Port numbers located in TCP header fields; 2-byte each (source, destination) Standard port numbers –Usual practice Nonstandard port numbers –Possible, but requires configuration of TCP TCP HTTP FTPSMTP …

Copyright 2005 John Wiley & Sons, Inc Application Layer Services

Copyright 2005 John Wiley & Sons, Inc Packetization and Reassembly receiver TCP IP FTP TCP IP FTP sender Application layer sees message as a single block of data Breaks a large message into smaller pieces (packetization) Delivers incoming packets as they arrive (e.g., Web pages) or to wait until entire message arrives (e.g., ) Puts them back together at the destination (reassembly) What size packet to use? Done through negotiations

Copyright 2005 John Wiley & Sons, Inc Setting up Virtual Connections A B SYN ACK 2 not busy Data 1 Data 2 Data 3 Data 4 FIN Requests a virtual circuit (TCP connection) and negotiates packet size with B Sends data packets one by one (in order) using continuous ARQ (sliding window) Closes virtual circuit

Copyright 2005 John Wiley & Sons, Inc Routing Implied by Transport Layer Connection Oriented (provided by TCP) –Setting up a virtual circuit (a TCP connection) TCP asks IP to route all packets in a message by using the same path (from source to destination) Packet deliveries are acknowledged Used by HTTP, SMTP, FTP Connectionless Routing (provided by UDP –Sending packets individually without a virtual circuit –Each packet is sent independently of one another (routed separately and can follow different routes and arrive at different times) QoS Routing (provided by RTP) –A special kind connection oriented routing with priorities

Copyright 2005 John Wiley & Sons, Inc UDP - User Datagram Protocol Protocol used for connectionless routing in TCP/IP suite (no acks, no flow control) Uses only a small packet header –Only 8 bytes containing only 4 fields: Source port Destination port Message length Header checksum Commonly used for control messages that are usually small, such as DNS, DHCP, RIP and SNMP.

Copyright 2005 John Wiley & Sons, Inc QoS - Quality of Service QoS parameters –Availability, Reliability, Timeliness Timeliness - timely delivery of packets –Packets be delivered within a certain period of time (to produce a smooth, continuous output –Required by some applications, especially real time applications (e.g., voice and video frames) –( doesn’t require this) QoS routing –Defines classes of service, each with a different priority: Real-time applications - highest A graphical file for a Web page - a lower priority - lowest (can wait a long time before delivery)

Copyright 2005 John Wiley & Sons, Inc Protocols Supporting QoS Asynchronous Transfer Mode (ATM) –A high-speed data link layer protocol TCP/IP protocol suite –Resource Reservation Protocol (RSVP) Sets up virtual circuits for general purpose real-time applications –Real-Time Streaming Protocol (RTSP) Sets up virtual circuits for audio-video applications –Real-Time Transport Protocol (RTP) Used after a virtual connection setup by RSVP or RTSP Adds a sequence number and a timestamp for helping applications to synchronize delivery Uses UDP (because of its small header) as transport IP RTSPRSVP UDP RTP

Copyright 2005 John Wiley & Sons, Inc Network Layer Functions Addressing –Each equipment on the path between source and destination must have an address –Internet Addresses –Assignment of addresses –Translation between network layer addresses and other addresses (address resolution) Routing –Process of deciding what path a packet must take to reach destination –Routing protocols

Copyright 2005 John Wiley & Sons, Inc Address TypeExampleExample Address Application Layer Network Layer Data Link Layer Types of Addresses IP address URL MAC address (4 bytes) 00-0C-00-F5-03-5A (6 bytes) Name Street # Apt # Analogy Try “ping”ing a URL; translation (corresponding IP address) will be given by the answer. These addresses must be translated from one type to another (for a message to travel from sender to receiver). This translation process is called address resolution.

Copyright 2005 John Wiley & Sons, Inc Assignment of Addresses Application Layer address (URL) –For servers only (clients don’t need it) –Assigned by network managers and placed in configuration files. –Some servers may have several application layer addresses Network Layer Address (IP address) –Assigned by network managers, or by programs such as DHCP, and placed in configuration files –Every network on the Internet is assigned a range of possible IP addresses for use on its network Data Link Layer Address (MAC address) –Unique hardware addresses placed on network interface cards by their manufacturers ( based on a standardized scheme) Servers have permanent addresses, clients usually do not

Copyright 2005 John Wiley & Sons, Inc Internet Addresses Managed by ICANN –Internet Corporation for Assigned Names and Numbers –Manages the assignment of both IP and application layer name space (domain names) Both assigned at the same time and in groups Manages some domains directly (e.g.,.com,.org,.net) and Authorizes private companies to become domain name registrars as well Example: Indiana University –URLs that end in.indiana.edu and iu.edu –IP addresses in the x.x range (where x is any number between 0 and 255)

Copyright 2005 John Wiley & Sons, Inc IPv4 Addresses 4 byte (32 bit) addresses –Strings of 32 binary bits Dotted decimal notation –Used to make IP addresses easier to understand for human readers –Breaks the address into four bytes and writes the digital equivalent for each byte Example:

Copyright 2005 John Wiley & Sons, Inc Classfull Adressing Class A Class B Class C Class D Class E 2^31 = 2 Billion addresses 2^30 = 1 Billion addresses 2^29 = 536 Million addresses 2^28 = 268 Million addresses ^28 = 268 Million addresses Net IDHost ID 7 bits24 bits Net ID Host ID 14 bits16 bits 21 bits8 bits Net ID Host ID

Copyright 2005 John Wiley & Sons, Inc IPv6 Addressing Need –IPv4 uses 4 byte addresses: Total of one billion possible addresses –IP addresses often assigned in (large) groups Giving out many numbers at a time  IPv4 address space has been used up quickly e.g., Indiana University: uses a Class A IP address space (65,000 addresses; many more than needed) IPv6 uses 16 byte addresses: –3.2 x addresses, a very large number –Little chance this address space will ever be used up

Copyright 2005 John Wiley & Sons, Inc Subnets Group of computers on the same LAN with IP numbers with the same prefix Assigned addresses that are 8 bits in length –For example: Subnet x –Computers in Business (x is between 0 & 255) Subnet x –Computers in CS department Assigned addresses could be more or less than eight bits in length –For example: If 7 bits used for a subnet Subnet 1: Subnet 2:

Copyright 2005 John Wiley & Sons, Inc Subnets: Example School of Business X GW School of Engineering X x GW Backbone

Copyright 2005 John Wiley & Sons, Inc Subnet Masks Used to make it easier to separate the subnet part of the address from the host part. Example –Subnet: x –Subnet mask: or in binary Example –Subnets: , –Subnet mask or, in binary:

Copyright 2005 John Wiley & Sons, Inc Dynamic Addressing Giving addresses to clients (automatically) only when they are logged in to a network –Eliminates permanent addresses to clients –When the computer is moved to another location, its new IP address is assigned automatically –Makes efficient use of IP address space –Example: A small ISP with several thousands subscribers Might only need to assign 500 IP addresses to clients at any one time Uses a server to supply IP addresses to computers whenever the computers connect to network

Copyright 2005 John Wiley & Sons, Inc Programs for Dynamic Addressing Bootstrap Protocol (bootp) Dynamic Host Control Protocol (DHCP) Different approaches, but same basic operations: –A program residing in a client establishes connection to bootp or DHCP server –A client broadcasts a message requesting an IP address (when it is turned on and connected) –Server (maintaining IP address pool) responds with a message containing IP address (and its subnet mask) –IP addresses can also be assigned with a time limit (leased IP addresses) When expires, client must send a new request

Copyright 2005 John Wiley & Sons, Inc Address Resolution Server Name Resolution –Translating destination host’s domain name to its corresponding IP address –e.g.,  ) –Uses one or more Domain Name Service (DNS) servers to resolve the address Data Link Layer Address Resolution –Identifying the MAC address of the next node (that packet must be forwarded t) –Uses Address Resolution Protocol (ARP)

Copyright 2005 John Wiley & Sons, Inc DNS - Domain Name Service Used to determine IP address for a given URL Provided through a group of name servers –Databases containing directories of domain names and their corresponding IP addresses Large organizations maintain their own name servers –smaller organizations rely on name servers provided by their ISPs When a domain name is registered, IP address of the DNS server must be provided to registrar for all URLs in this domain –Example: Domain name: indiana.edu URLs: w ww.indiana.edu, abc.indiana.edu

Copyright 2005 John Wiley & Sons, Inc How DNS Works Desired URL in client’s address table: –Use the corresponding IP address –Each client maintains a server address table containing URLs used and corresponding IP addresses Desired URL not in client’s address table: –Use DNS to resolve the address –Sends a DNS request packet to its local DNS server –URL in Local DNS server Responds by sending a DNS response packet back to the client

Copyright 2005 John Wiley & Sons, Inc How DNS Works (Cont.) URL NOT in Local DNS server –Sends DNS request packet to the next highest name server in the DNS hierarchy –Usually the DNS server at the top level domain (such as the DNS server for all.edu domains) –URL NOT in the name server Sends DNS request packet ahead to name server at the next lower level of the DNS hierarchy

Copyright 2005 John Wiley & Sons, Inc How DNS Works Asks for a web page on Indiana University’s server Client computer DNS Server DNS Request LAN Internet DNS Request DNS Server Root DNS Server for.EDU domain University of Toronto Indiana University DNS Request DNS Response

Copyright 2005 John Wiley & Sons, Inc MAC Address Resolution Problem: –Unknown MAC address of the next node (whose IP address known) Solution: –Uses Address Resolution Protocol (ARP) Operation –Broadcast an ARP message to all nodes on a LAN asking which node has a certain IP address –Host with that IP address then responds by sending back its MAC address –Store this MAC address in its address table –Send the message to the destination node Example of a MAC address: 00-0C-00-F5-03-5A

Copyright 2005 John Wiley & Sons, Inc Routing Process of identifying what path to have a packet take through a network from sender to receiver Routing Tables –Used to make routing decisions –Shows which path to send packets on to reach a given destination –Kept by computers making routing decisions Routers –Special purpose devices used to handle routing decisions on the Internet –Maintain their own routing tables Dest. B C D E F G Next B D B

Copyright 2005 John Wiley & Sons, Inc Routing Example Dest. B C D E F G Next B D B Routing Table for A Possible paths from A to G: ABCG ABEFCG ADEFCG ADEBCG B Each node has its own routing table A

Copyright 2005 John Wiley & Sons, Inc Types of Routing Centralized routing –Decisions made by one central computer –Used on small, mainframe-based networks Decentralized routing –Decisions made by each node independently of one another –Information need to be exchanged to prepare routing tables –Used by Internet

Copyright 2005 John Wiley & Sons, Inc Types of Decentralized Routing Static routing: –Uses fixed routing tables developed by network managers Each node has its own routing table Changes when computers added or removed –Used on relatively simple networks (with few routing options that rarely change) Dynamic routing (aka. Adaptive routing): –Uses routing tables (at each node) that are updated dynamically –Based on routing condition information exchanged between routing devices

Copyright 2005 John Wiley & Sons, Inc Dynamic Routing Algorithms Distance Vector –Uses the least number of hops to decide how to route a packet –Used by Routing Information Protocol (RIP) Link State –Uses a variety of information types to decide how to route a packet (more sophisticated) e.g., number of hops, congestion, speed of circuit –Links state info exchanged periodically by each node to keep every node in the network up to date –Provides more reliable, up to date paths to destinations –Used by Open Shortest Path First (OSPF) B A C D E F G Ex: From A to G  ABCG

Copyright 2005 John Wiley & Sons, Inc Routing Protocols Used to exchange info among nodes for building and maintaining routing tables Autonomous System (AS) –A network operated by an organization (e.g., Indiana U.) –Protocols classified based on autonomous systems Types of Routing Protocols –Interior routing protocols (RIP, OSPF, EIGRP, ICMP) Operate within a network (autonomous system) Provide detailed info about each node and paths –Exterior routing protocols (BGP) Operate between networks (autonomous systems)

Copyright 2005 John Wiley & Sons, Inc Routing Information Protocol (RIP) A dynamic distance vector interior routing protocol Once popular on Internet; now used on simple networks Operations: –Manager builds a routing table by suing RIP –Routing tables broadcast periodically (every minute or so) by all nodes –When a new node added, RIP counts number of hops between computers and updates routing tables

Copyright 2005 John Wiley & Sons, Inc Open Shortest Path First (OSPF) A dynamic link state interior routing protocol Became more popular on Internet –More reliable paths Incorporates traffic and error rate measures –Less burdensome to the network Only the updates sent (not entire routing tables) and only to other routers (no broadcasting)

Copyright 2005 John Wiley & Sons, Inc Other Interior Routing Protocols Enhanced Interior Gateway Routing Protocol (EIGRP) –A dynamic link state protocol (developed by Cisco) –Records transmission capacity, delay time, reliability and load for all paths –Keeps the routing tables for its neighbors and uses this information in its routing decisions as well Internet Control Message Protocol (ICMP) –Simplest and most basic –An error reporting protocol (report routing errors to message senders) –Limited ability to update routing tables

Copyright 2005 John Wiley & Sons, Inc Exterior Routing Protocols Border Gateway Protocol (BGP) –Used to exchange routing info between autonomous systems –Based on a dynamic distance vector algorithm –Far more complex than interior routing protocols –Provide routing info only on selected routes (e.g., preferred or best route) Privacy concern Too many routes; can’t maintain tables of every single rout

Copyright 2005 John Wiley & Sons, Inc Internet Routing using BGP, OSPF and RIP Router 1 Router 4 Router 3 OSPF Designated Router Border Router Router 2 Autonomous System A (using OSPF) Router 1 Router 4 Router 3 Border Router Router 2 Autonomous System B (using RIP) Router 1 Router 4 Border Router Router 3 Router 2 Autonomous System C (using OSPF) BGP OSPF Designated Router Autonomous System E Autonomous System D Autonomous System F BGP Router 5

Copyright 2005 John Wiley & Sons, Inc Multicasting Casting –Unicast message: one computer  another computer –Broadcast message: one computer  all computers in the network –Multicast message: one computer  a group of computers (e.g., videoconference) Internet Group Management Protocol (IGMP) –Provides a way for a computer to report its multicast group membership to adjacent routers –A special IP address assigned to identify the group –Routing node sets MAC address to a matching MAC address –When multicast session ends, IGMP sends a message to the organizing computer( or router) to remove multicast group

Copyright 2005 John Wiley & Sons, Inc Sending Messages using TCP/IP Required Network layer addressing information –Computer’s own IP address –Its subnet mask To determine what addresses are part of its subnet –Local DNS server’s IP address To translate URLs into IP addresses –IP address of the router (gateway) on its subnet To route messages going outside of its subnet Obtained from a configuration file or provided by a DHCP server –Servers also need to know their own application layer addresses (domain names)

Copyright 2005 John Wiley & Sons, Inc TCP/IP Configuration Information

Copyright 2005 John Wiley & Sons, Inc TCP/IP Network Example

Copyright 2005 John Wiley & Sons, Inc Case 1a: Known Address, Same Subnet Case: –A Client ( ) requests a Web page from a server (www1.anyorg.com) –Client knows the server’s IP and Ethernet addresses Operations (performed by the client) –Prepare HTTP packet and send it to TCP –Place HTTP packet into a TCP packet and sent it to IP –Place TCP packet into an IP packet, add destination IP address, –Use its subnet mask to see that the destination is on the same subnet as itself –Add server’s Ethernet address into its destination address field, and send the frame to the Web server

Copyright 2005 John Wiley & Sons, Inc Case 1b: HTTP response to client Operations (performed by the server) –Receive Ethernet frame, perform error checking and send back an ACK –Process incoming frame successively up the layers (data link, network, transport and application) until the HTTP request emerges –Process HTTP request and sends back an HTTP response (with requested Web page) –Process outgoing HTTP response successively down the layers until an Ethernet frame is created –Send Ethernet frame to the client Operations (performed by the client) –Receive Ethernet frame and process it successively up the layers until the HTTP response emerges at browser

Copyright 2005 John Wiley & Sons, Inc Case 2: Known Address, Different Subnet Similar to Case 1a Differences –Use subnet mask to determine that the destination is NOT on the same subnet –Send outgoing frames to the local subnet’s GW –Local gateway operations Receive the frame and remove the Ethernet header Determine the next node (via Router Table) Make a new frame and send it to the destination GW –Destination gateway operations Remove the header, determine the destination (by destination IP address) Place the IP packet in a new Ethernet frame and send it to its final destination.

Copyright 2005 John Wiley & Sons, Inc Case 3: Unknown Address Operations (by the host) –Determine the destination IP address Send a UDP packet to the local DNS server Local DNS server knows the destination host’s IP address –Sends a DNS response back to the sending host Local DNS server does not know the destination IP address –Send a second UDP packet to the next highest DNS host, and so on, until the destination host’s IP address is determined –Follow steps in Case 2

Copyright 2005 John Wiley & Sons, Inc TCP Connections Before any data packet is sent, a connection is established –Use SYN packet to establish connection –Use FIN packet to close the connection Handling of HTTP packets –Old version: a separate TCP connection for each HTTP Request –New version: Open a connection when a request (first HTTPP Request) send to the server Leave the connection open for all subsequent HTTP requests to the same server Close the connection when the session ends

Copyright 2005 John Wiley & Sons, Inc TCP/IP and Layers Host Computers –Packets move through all layers Gateways, Routers –Packet moves from Physical layer to Data Link Layer through the network Layer At each stop along the way –Ethernet packets is removed and a new one is created for the next node –IP and above packets never change in transit (created by the original sender and destroyed by the final receiver)

Copyright 2005 John Wiley & Sons, Inc Message Move Through Layers

Copyright 2005 John Wiley & Sons, Inc Implications for Management Most organizations moving toward a single standard, TCP/IP –Decreased cost of buying and maintaining network equipment –Decreased cost of training networking staff Telephone companies (having large non- TCP/IP networks) moving toward TCP/IP –Significant financial implications for telcos –Significant financial implications of networking equipment manufacturers

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